Origin of Blue-Green Emission in α-Zn₂P₂O₇ and Local Structure of Ln³⁺ Ion in α-Zn₂P₂O₇:Ln³⁺ (Ln = Sm, Eu): Time-Resolved Photoluminescence, EXAFS, and DFT Measurements

Abstract

Considering the fact that pyrophosphate-based hosts are in high demand for making highly efficient luminescence materials, we doped two visible lanthanide ions, viz. Sm and Eu, in ZnPO. Interestingly, it was oberved that pure ZnPO displayed blue-green dual emission on irradiation with ultraviolet light. Emission and lifetime spectroscopy shows the presence of defects in pyrophosphate samples which are responsible for such emission. DFT calculations clearly pinpointed that the electronic transitions between defect states located at just below the conduction band minimum (arises due to V and V defects) and valence band maximum, as well as impurity states situated in the band gap, can lead to dual emission in the blue-green region, as is also indicated by emission and lifetime spectra. X-ray absorption near edge spectroscopy (XANES) shows the stabilization of europium as well as samarium ion in the +3 oxidation state in α-ZnPO. The fact that α-ZnPO has two different coordination numbers for zinc ions, i.e. five- and six-coordinate, the study of dopant ion distribution in this particular matrix will be an important step in realizing a highly efficient europium- and samarium-based red-emitting phosphor. Time resolved photoluminescence (TRPL) shows that both of these ions are heterogeneously distributed between five- and six-coordinated Zn sites and it is the six-coordinated Zn site which is the most favorable for lanthanide ion doping. Extended X-ray absorption fine structure (EXAFS) measurements also suggested that a six-coordinated zinc ion is the preferred site occupied by trivalent lanthanide ions, which is in complete agreement with TRPL results. It was observed that there is almost complete transfer of photon energy from ZnPO to Eu, whereas this transfer is inefficient and almost incomplete in case of Sm, which is indeed important information for the realization of pyrophosphate-based tunable phosphors.

Achieving Bright Blue and Red Luminescence in Ca2SnO4 through Defect and Doping Manipulation

“…Blue emission in CSO can also be clearly seen from the CIE index diagram shown as Figure S2 of SI. Based on an earlier report on inorganic oxides, such visible emission is attributed to the presence of defects in the band gap of materials, viz., cation vacancies, anion vacancies, interstitials, etc. ,, Such blue emission has been previously observed in 1D chain-type structures such as Sr 2 CeO 4 and Ca 2 Sn 1– x Ce x O 4 wherein authors have attributed the same to oxygen-to-metal charge transfer. , Blue emission could also be observed in other hosts such as CaZrO 3 , SrTiO 3 , Nd 2 Zr 2 O 7 , Zn 2 P 2 O 7 , Pr 2 Sn 2 O 7 , SrZrO 3 , Sr 2 SnO 4 , MgAl 2 O 4 , etc. ,,,− wherein the origin of such emission is attributed to OVs. In the literature, we could find only two reports wherein authors have carried out emission spectroscopy of CSO. , Both the groups have contradictory remarks about this emission; Yu et al have attributed the same simply to OVs, whereas Gao et al have ascribed the same to O 2– → Sn 4+ ligand-to-metal charge transfer, and in fact, they have pointed out that OVs quench the blue emission in CSO.…”

“…have carried out very interesting work with CSO and found that lanthanide-doped CSO can be an excellent candidate for short-wave infrared (SWIR) persistence luminescence. Local site occupancy of dopant ions in a host matrix with multiple available sites such as Ca 2+ and Sn 4+ in this case is very critical in understanding and optimizing the performance of the optical materials as it dictates the branching ratio, color coordinates, and color temperature. ,,− Yamane et al and Ishigaki et al have reported that Eu 3+ ions occupy both Ca 2+ and Sn 4+ sites in CSO based on their experimental results only. Fujichimi et al and Muto et al have also reported that Eu 3+ ions occupy both Ca 2+ and Sn 4+ sites in CSO based on their electron channeling microanalysis, but occupancy is slightly biased toward occupying larger-sized cations in the Y 3+ -codoped sample.…”

“…The nature of OVs (neutral, singly ionized, or doubly ionized), which is responsible for the same, has not been discussed at all. Such studies require extensive density functional theory (DFT) calculations for calculating the density of states for different configurations of OVs as has been reported in other oxide materials. − Doping is a very efficient strategy to harness the full potential of this excellent oxide host for luminescence applications as it will help in understanding the dynamics of host-to-dopant energy transfer, excited state lifetime, etc. Creation of V Ca ″ (calcium vacancies with two negative charges) and Sm Ca ■ (a samarium ion occupying a calcium ion with one positive charge and is called as antisite defect) as a result of aliovalent substitution of Sm 3+ at the Ca 2+ sites in CSO was shown to result in afterglow emission for around 1 h. , Also, doping of Eu 3+ ions leads to generation of several holes and donors, which leads to afterglow emission in CSO .…”

Thermally Induced Disorder–Order Phase Transition of Gd2Hf2O7:Eu3+ Nanoparticles and Its Implication on Photo- and Radioluminescence

“…33,56−61 Normally, the short lifetime is attributed to asymmetric environment as f–f transition becomes relaxed, and the long lifetime is mostly attributed to symmetric environment as f–f transition is La Porte forbidden. 59,62−65 The biexponential behavior can also arise due to other reasons such as the presence of defects, energy transfer, and so forth. This phenomenon has been substantiated with theoretical calculations in the next section.…”

Modulating the optical and electrical properties of oxygen vacancy-enriched La2Ce2O7:Sm3+ pyrochlore: role of dopant local structure and concentration

“…In cases where a mismatch is found, there is incomplete or inefficient energy transfer, such as in SrZrO 3 :Tb 3+ , MgAl 2 O 4 :Eu 3+ and Zn 2 P 2 O 7 :Sm 3+ . 17–19…”